Solid electrolytic capacitors with conductive polymers as the cathode materials have been widely used in the electronics industry due to their advantageously low equivalent series resistance (ESR) and “non-burning/non-ignition” failure mode. Various types of conductive polymers including polypyrrole, polyaniline, and poly(3,4-ethyldioxythiophene) (PEDOT) are applied to electrolytic capacitors as a cathode material when valve metals such as Ta, Al, and Nb as well as conductive oxides such as ceramic NbO, are used as the anode.
In a manufacturing process to produce conductive polymer based valve metal capacitors, Ta powder, for example, is mechanically pressed to form Ta metal pellets, which are subsequently sintered at high temperature under vacuum. The sintered pellets are anodized in an electrolyte solution to form a dielectric layer (Ta2O5) on the anode surface. Following that, multiple layers of a conductive polymer, such as poly 3,4-ethylenedioxythiophene (PEDOT), are laid down by a multiple dipping polymerization process. During the polymerization process, an oxidant solution, such as iron (III) p-toluenensulfonate solution in a solvent, is first applied onto the anodes. It is then followed by the application of a liquid monomer or monomer solution as disclosed by D Wheeler, et al. in U.S. Pat. No. 6,136,176 and by R. Hahn, et al., in U.S. Pat. No. 6,334,966. A polymer layer will form on the surface of the dielectric as the result of polymerization reaction. The polymer coated anodes are washed to remove excessive reactants and byproducts. This polymerization step may be repeated multiple times to achieve a desired thickness of the conductive polymer layer. The conductive polymer layer should be reasonably robust in order to protect the anodes from potential mechanical damages in the post-polymerization process and from direct contact with carbon and silver layers, which are subsequently applied to provide connection between conductive polymer cathode and the outside circuitry. The carbon and silver coated anodes are then encapsulated, aged and tested to complete the manufacturing process. An illustrative structure of a conductive polymer based capacitor is illustrated in FIG. 1.
In FIG. 1, the capacitor has an anode, 1, such as a tantalum anode. An anode wire, 2, such as a tantalum wire extends from the anode and is in electrical contact with a leadframe, 8, such as by a weld, 3. A dielectric is on the surface of the anode. Coated on the surface of the dielectric is a conductive polymer, 5. A carbon coating, 4, and silver paint, 7, provide adhesion and conduction to a cathode lead preferably through a silver adhesive, 10. A solderable coating, 9, on the lead frame is provided to increase adhesion during mounting to a substrate or the like. A washer, 6, protects the anode wire during layer buildup.
The ESR characteristics of a conductive polymer based capacitor are heavily influenced by the quality of conductive polymer. It is highly desirable that the structure of the conductive polymer has a high degree of electron delocalization, or conjugation, as in an alternate single and double bond structure or in an aromatic, which provides the foundation for achieving high conductivity. Also, a dense, robust polymer layer is essential for low ESR capacitors which are subjected to the thermal mechanical stresses when they are mounted onto a circuit board via surface mounting. ESR varies with the size of the anode in the finished capacitor. For the purposes of the present invention the ESR is defined based on a V-case anode with dimensions of 4.9 mm×3.25 mm×1.7 mm, wherein low ESR is less defined to be less than about 50 milliohms, preferably less than about 25 milliohms, and most preferably less than about 10 milliohms. A polymerization process that consistently produces such quality of polymer is highly desirable.
It is known that the formation of conductive polymer follows an oxidative coupling mechanism. The monomers are oxidized by oxidants such as ferric salts to form charged radicals, which then couple with each other to become dimers. These dimers will be further oxidized to form higher molecular segments via similar steps resulting in the formation of polymers. An example of the polymer, PEDOT, is provided in FIG. 2.
Through diligent research the present inventors have determined elevated levels of non-conjugated dihydrothiphene in the monomer bath can lead to significant degradation in the performance characteristics in the finished product. The degradation is realized in an increase in ESR of the final capacitor. Previously, the mechanism for this degradation was not accurately characterized. In an effort to insure adequate capacitor properties it was the standard practice of skilled artisans to frequently replace the monomer solution. This leads to excessive waste and inefficient use of human resources.
Through diligent research the present inventors have determined the mechanism of degradation and have provided an improvement in the monomer solution, resulting polymer film, and a surprising improvement in the resulting capacitor by mitigating the effects of the degradation mechanism.